A reciprocating working machine for trimming garden trees and plants, such as a trimming machine (or hedge trimmer), a swing-type hedge trimmer and a lawn mower, is known. In the trimming machine, for example, a pair of blades (for example, upper and lower blades) with cutting edges are reciprocated relative to each other in opposite directions along a longitudinal direction. Further, in order to convert a rotary motion of a rotary power machine to a reciprocating motion of the pair of blades, a pair of eccentric cams ((or example, upper and lower eccentric cams) are eccentrically rotated by the rotary power machine and are slidably fitted into elongated apertures of the respective blades (For example, see Patent Publication 1). In order to move the pair of blades in the opposite directions, the pair of eccentric cams are arranged at a 180-degree phase from each other.